Is it possible to set the framerate of an animated image texture?
As far as I know you cannot set the framerate in the Node Editor differently for specific sequences (better answers welcome). If I understand the question correctly, you need your project to keep the 24 fps, right? If not, just set it to 18 fps if that's what you need for the image sequence.
If you instead want to play it slower than the project framerate, you could load the sequence into the Video Editor and add an Effect Strip &gt; Speed Control to it.
Set the Speed Factor to 18/24 = 0.75 and stretch the sequence to a rounded 85 frames. Now render the 85 frames as a new image sequence so you can use it in your 24 fps project and it will be about 3.5 seconds long like it would have been at 18 fps. Unfortunately this results cross-fading images that are put inbetween the existing.
This is just a workaround in lack of a better solution. Of course you wouldn't have to re-render if you would only be working in the Video Editor combining multiple image sequences with different speeds - in this case you just keep them adjusted by the Speed Control Effect Strip.